IBM said in a statement that it had taken great care to comply with privacy principles.

But one digital rights group said IBM's actions represented a "huge threat" to people's privacy.

"None of the people I photographed had any idea their images were being used in this way," a photographer told NBC News.

Photos selected by IBM were listed under a Creative Commons licence, which generally means the images can be widely used with only a small number of restrictions.

In a paper published online about the work, IBM researchers describe in detail the steps taken to analyse people's faces, including taking measurements of the distance between individuals' facial features.

"Many of these measures can be reliably estimated from photos of frontal faces, using 47 landmark points of the head and face," the researchers wrote.

It was these measurements, rather than the photos themselves, that IBM has compiled into its own dataset.

'Don't be creepy'

Such data can be used to help artificial neural networks better distinguish between faces, so that individuals can be recognised in different images.

By using large datasets such as this, technology companies hope to make their facial-recognition systems more accurate.

"We take the privacy of individuals very seriously and have taken great care to comply with privacy principles," said IBM in a statement.

"Individuals can opt out of this dataset."

However, opting out would only be an option if an individual were aware that their data had been used in the first place.

Digital rights group Privacy International said IBM had been wrong to use the photos without direct consent from those pictured.